Bias changes with tube angle?

[vintasonic]

Sound weird but this is the first time I noticed.

I have an amp on my bench that had some intermittent problems associated with the output tubes and the bias - which I seem to have sorted out.  However, when I was adjusting the bias I noticed that one of the tubes bias current changed pretty dramatically when I changed the chassis position from flat to sideways - from about 32 ma to 22 ma. When I returned it to the flat position the bias returned to 32ma. I swapped tube sockets and it followed the tube so it's definitely a problem with the tube.

I have a bunch of used tubes I keep around for testing and tried the same thing with some of them.  Sure enough, at least one other one exhibited the same problems.  I threw another used pair in and didn't see any change, so it's something to do with the specific tube.

Can anybody clue me in to what's going on?  I understand there is some mechanical thing happening, some element must be moving but I can't imagine what might cause it to act that way without seeing other, bigger problems.  I have replaced hundreds of output tubes and don't recall ever noticing this before

FWIW, EL34s in a Blackstar HT60